In general, as shown in FIG. 1, a liquid crystal display panel is composed of the cross-connected data lines (D1, D2, D3, . . . Dy) and the scan lines (G1, G2, . . . Gx). Each data and scan line pair can be used to control a display cell. For example, data line D1 and scan line G1 can be used to control the display cell 100. As shown in FIG. 1, the equivalent circuit of display cell 100 (the same for other display cells) includes a thin film transistor 10 for control, a storage capacitor Cs and a liquid crystal capacitor Clc constructed by the display electrode and the common electrode. The gate and the drain of the thin film transistor 10 are connected to scan line G1 and data line D1, respectively. The video signal carried by the data line D1 can be written to the display cell 100 by controlling the conducting state of the thin film transistor 10 according to the scan signal carried by the scan line G1.
Scan driver 30 sends out the scan signal on the scan line G1, G2, . . . Gx sequentially, according to scan control signals. When one of the scan lines is scanned, the thin film transistors corresponding to this scanned line are turned on and the thin film transistors corresponding to other scan lines are turned off. When the thin film transistors of the display cells in a row are turned on, data driver 20 sends a corresponding video signal (gray level) to data lines D1, D2, and Dy. When scan driver 30 finishes scanning the scan lines, the display of a single video frame is done. The scanning of the scan lines described above is performed repeatedly, thereby displaying subsequent video frames.
To prevent the liquid crystal molecules from being subjected to a voltage bias of single polarity and therefore shortening the life of the liquid crystal molecules, a single display cell in the general TFT-LCD is driven by video signals of opposite polarities in the odd-numbered video frames and even-numbered video frames. There are four driving schemes that achieve the above-described requirement, including frame inversion, row inversion, column-inversion and dot-inversion.
In the row inversion, as shown in FIG. 2, the polarity of voltage applied to the pixel electrodes is reversed at every scan line (row). In the column inversion, as shown in FIG. 3, the polarity of voltage applied to the pixel electrodes is reversed at every data line (column). In the dot inversion method, as shown in FIG. 4, the polarity of voltage is reversed at adjacent scan lines or data lines.
In the row and column inversion method, a flicker problem occurs. The reason is given as follows. When a scan line signal is “HIGH,” all the TFTs connected to the scan line are turned on, and the video signals are sent to the pixel electrodes from the drain electrodes connected to the data lines. Then, the liquid crystal is driven by the voltage difference between the pixel electrode and the common electrode. When the scan line signal is “LOW”, all the TFTs connected to the scan line are turned off. At that time, the voltage of the video signal applied to the pixel electrodes remains in the pixel electrode, and the display image is maintained. However, the stored voltage in the pixel electrode is reduced by ΔV by coupling capacitors (Cgs), which are formed between the scan lines and data lines. Since the voltage in the pixel electrodes is not constant, the display has a flicker problem.
Although the dot-inversion method can reduce the flicker problem, this method has to use a constant common voltage. In other words, the common voltage, such as 0 volt, and two opposing voltages, such as +2 volt and −2 volt, are used to form a positive polarity and a negative polarity for the same gray level so that it is possible to output a voltage two times greater than the row inversion driving process. Moreover, a larger driver area is required in the dot-inversion method. As a result, this dot-inversion driving method causes an increase in the cost of a driver and larger power consumption than with the row inversion driving system.